Epithermal Beam Development at the BMRR: Dosimetric Evaluation

Abstract

The utilization of an epithermal-neutron beam for neutron capture therapy (NCT) is desirable because of the increased tissue penetration relative to a thermal-neutron beam. Over the past few years, modifications have been and continue to be made at the Brookhaven Medical Research Reactor (BMRR) to produce an optimal epithermal beam by changing filter components. An optimal incident epithermal beam should contain the minimum possible fast-neutron component and no thermal neutrons. Recently, a new moderator for the epithenmal beam was installed at the epithermal port of the BMRR. With the installation of this moderator, an optimal beam has been realized [1]. This new moderator is a combination of alumina (Al2O3) bricks and aluminum (Al) plates. A 0.51-mm thick cadmium (Cd) sheet has reduced the thermal-neutron intensity drastically. Furthermore, an 11.5-cm thick bismuth (Bi) plate installed at the port surface has reduced the gamma-dose component to negligible levels. In order to compare various filter configurations for best optimization [2], the following parameters have been measured on the beam axis, directly in front of the epithermal port: 1) Thermal-neutron fluence rate free in air 2) Epithermal-neutron fluence rate free in air 3) Fast-neutron fluence rate free in air 4) Thermal-neutron fluence rate in a polyethylene cylindrical head phantom as a function of distance along the axis of the phantom 5) Fast-neutron dose rate in soft tissue, free in air, and 6) Gamma-dose rate in soft tissue, free in air.